drums for soundtrack :D need help

sorry, didn't really know where this would fit. my friends asked me to create a soundtrack for a computer game they're making. i'm completely ready with guitar and bass but drums i have a problem with. i only play guitar and bass (hence i posted on UG) and only have a little knowledge of drums but i have no drum set. is there any free software that would allow me to create the drums part of the track? im not looking for techno beats here, im looking to emulate real drums used for rock, metal and the like. any suggestions? any help on the matter would be highly appreciated. thanks guys!